Propeller health monitoring

ABSTRACT

A method of monitoring the health of an aircraft propeller whilst the propeller is in operation, the propeller having a plurality of blades extending radially outwardly from a central axis extending through the propeller and a propeller drive shaft, is provided. The method includes obtaining measurements representative of strain in the propeller drive shaft using multiple primary strain sensors, each primary strain sensor providing respective measurements representative of strain. The primary strain sensors are located around a circumference of the drive shaft of the propeller, and each strain sensor is located such that it crosses a plane defined by the radial direction of a blade and the central axis, the plane being bounded by the central axis. A corresponding propeller health monitoring system, an aircraft propeller comprising the system and an aircraft comprising the propeller are also provided.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.17305528.6 filed May 10, 2017, the entire contents of which isincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to the technical area of health monitoring ofpropellers for providing a warning or an indication that maintenance isrequired. In particular, the field of the disclosure lies in the area ofaircraft engine propellers.

BACKGROUND OF THE DISCLOSURE

It is known in the art to monitor the health of propeller blades on anaircraft to assess if maintenance work needs to be carried out. Thisprevents unnecessary maintenance checks being undertaken and alsoprovides early warnings of damage.

U.S. Pat. No. 9,240,083 B2 discloses a general method of monitoring arotor for faults. The loads on a rotor shaft are measured to obtain ameasured signal. A residual is calculated between this measured signaland a virtual estimated signal. The residual is subsequently comparedwith a categorical model, and an output representative of a rotor faultis obtained.

The present disclosure aims to provide improved methods and apparatusesfor propeller health monitoring.

SUMMARY OF THE DISCLOSURE

In a first aspect, the disclosure provides a method of monitoring thehealth of an aircraft propeller whilst the propeller is in operation,the propeller having a plurality of blades extending radially outwardlyfrom a central axis extending through the propeller and a propellerdrive shaft, the method comprising: obtaining measurementsrepresentative of the strain in the propeller drive shaft using multipleprimary strain sensors, each primary strain sensor providing respectivemeasurements representative of strain; wherein the primary strainsensors are located around a circumference of the drive shaft of thepropeller; and wherein each strain sensor is located such that itcrosses a plane defined by the radial direction of a blade and thecentral axis, the plane being bounded by the central axis.

It will be appreciated that the primary strain sensor can therefore beunderstood as corresponding to the blade at which it is located bycrossing a plane defined by the radial direction of the blade and thecentral axis, the plane being bounded by the central axis. Conversely,the blade may be considered as corresponding to the primary strainsensor which is located such that it crosses a plane defined by theradial direction of the blade and the central axis, the plane beingbounded by the central axis.

Strain sensors may also be known as strain gauges.

Obtaining measurements representative of strain may comprise obtainingvoltage values from strain sensors that are representative of strain,for example from a full, half or quarter-bridge strain gauge comprisingfoil sensors. The measurements representative of strain may comprisevoltage values.

The step of obtaining measurements representative of strain may comprisemeasuring the strain. The measurements representative of strain maycomprise strain measurements.

The measurements representative of strain may be obtained over time,e.g. the measurements may be made continuously or periodically over aperiod of time. Multiple strain measurements over a period of timeenable bending moment values to be determined over a period of time, andthus the steady (i.e. average) bending moment to be determined.

In embodiments, the method may further comprise obtaining measurementsrepresentative of strain in the propeller drive shaft using multiplesecondary strain sensors, each secondary strain sensor being locatedaround the circumference of the drive shaft diametrically opposite to arespective primary strain sensor and forming a sensor pair therewith.

In embodiments, the method may further comprise: determining arespective steady bending moment of the drive shaft corresponding toeach primary strain sensor using the respective measurementsrepresentative of strain obtained by each primary strain sensor or usingthe respective measurements representative of strain obtained by eachstrain sensor pair.

The step of determining a respective steady bending moment of the driveshaft may comprise converting each measurement representative of straininto a bending moment value. This may be done by calculating the bendingmoment from the measurements representative of strain. A calibration maybe made between measurements representative of strain and bending momentin order to find a constant value to convert strain to bending moment,without the need for a full bending moment calculation.

The step of determining a respective steady bending moment of the driveshaft may include utilising a first algorithm to calculate the timetaken for one revolution of the propeller, given by (RPM/60)^−1. Then, asecond algorithm may be utilised to record the maximum and minimumbending moment determined utilising the measurements representative ofstrain from each primary sensor (or sensor pair) in each revolution. Athird algorithm may be used to calculate the steady bending moment (SBM)corresponding to each strain sensor in each revolution by taking theaverage of the recorded maximum and minimum bending moments, i.e.(max+min)/2. It will be appreciated that in order to determine thesteady bending moment in this way, the measurements representative ofstrain should be obtained over a period of time.

The magnitude of the calculated steady bending moments may be comparedto a threshold. Furthermore, the method may comprise establishing thatthe health of the propeller may be impaired if the magnitude of acalculated steady bending moment of the drive shaft is above athreshold. It may also include indicating an alert for maintenance if itis established that the health of the propeller may be impaired.

The magnitude of the calculated steady bending moments may be comparedto one another. Furthermore, the method may comprise establishing thatthe health of the propeller may be impaired if the magnitude of one ofthe steady bending moments of the drive shaft is outside of a toleranceof the other steady bending moments of the drive shaft. It may alsoinclude indicating an alert for maintenance if it is established thatthe health of the propeller may be impaired.

In embodiments, the method may further comprise for a propeller havingan odd number of blades, identifying a damaged blade by: identifying theblade corresponding to the primary sensor or sensor pair which providedthe measurements representative of strain which has led to a steadybending moment being calculated which has a magnitude above thethreshold and/or which is outside of the tolerance of the other steadybending moments of the drive shaft. An alert for maintenance of theidentified blade may be indicated.

In embodiments, the method may further comprise, for a propeller havingan even number of blades, identifying which two diametrically opposedblades may include at least one damaged blade, by: identifying theblades corresponding to the primary sensor or sensor pair which providedthe measurements representative of strain which has led to a steadybending moment being calculated which has a magnitude above thethreshold and/or which is outside of the tolerance of the other steadybending moments of the drive shaft. An alert for maintenance of theidentified blades may be indicated.

By a blade “corresponding to” the primary sensor or sensor pair will beunderstood as meaning the blade having the primary strain sensor locatedsuch that it crosses a plane defined by the radial direction of the saidblade and the central axis, the plane being bounded by the central axis.

In embodiments, the strain sensors are full bridge strain gauges.

The disclosure further provides a system configured to perform a methodfor monitoring aircraft propeller health according to any of theembodiments described above.

The disclosure further provides a propeller health monitoring systemcomprising: a plurality of primary strain sensors or pairs of primaryand secondary strain sensors, the primary strain sensors or strainsensor pairs being configured to provide measurements representative ofstrain in a drive shaft of a propeller; and a processor configured tocarry out the determining, comparing and establishing steps as describedin any of the above embodiments. Moreover, the disclosure provides anaircraft propeller comprising such a propeller health monitoring system,wherein: the propeller has a plurality of blades extending radiallyoutwardly from hub arms of a propeller hub, which in turn extendradially outwardly from a central axis extending through the propellerand a propeller drive shaft; the primary strain sensors or pairs ofprimary and secondary strain sensors are arranged around a circumferenceof the drive shaft of the propeller; each primary strain sensor islocated such that it crosses a plane defined by the radial direction ofa blade and the central axis, the plane being bounded by the centralaxis; and in the case in which strain sensor pairs are provided, eachsecondary strain sensor in the strain sensor pair is located around thecircumference of the drive shaft diametrically opposite to itscorresponding primary strain sensor. The processor may be integratedinto a FADEC of the aircraft or in the nacelle and the strain sensorsare configured to transmit the measured strain to the processor viatelemetry, Wi-Fi, or a slip ring.

Also provided is an aircraft comprising such an aircraft propeller.

In various embodiments as described, it is established that the healthof the propeller may be impaired, e.g. that damage may have occurred.“May be” is used since the methods provide an indication, and notnecessarily 100% certainty that the health is impaired. However, inembodiments, it may be said that the methods include establishing thatthe health of the propeller is impaired.

It will be readily appreciated by the skilled person that the variousoptional and preferred features of embodiments of the disclosuredescribed above may be applicable to all the various aspects andembodiments of the disclosure.

BRIEF DESCRIPTION OF THE FIGURES

Preferred embodiments of the present disclosure will now be described byway of example only and with reference to the accompanying drawings, inwhich:

FIG. 1 shows schematically a first embodiment of a first propellerhealth monitoring arrangement for a propeller having four blades;

FIG. 2 shows a cross section of the drive shaft of FIG. 1, taken alongthe line A-A, the drive shaft having four sensors mounted thereto;

FIG. 3 is a graph illustrating the shaft bending moments measured by twoof the sensors using the arrangement of FIGS. 1 and 2 for a propeller inwhich the blades corresponding to the sensors are healthy (i.e.undamaged);

FIG. 4 is a graph illustrating the shaft bending moments measured by twoof the sensors using the arrangement of FIGS. 1 and 2, for a propellerin which a blade and the blade diametrically opposite thereto arehealthy (undamaged) and at least one is damaged;

FIG. 5 shows schematically a second embodiment of a first propellerhealth monitoring arrangement for a propeller having three blades;

FIG. 6 shows a cross section of the drive shaft of FIG. 5, taken alongthe line B-B, the drive shaft having three sensors mounted thereto;

FIG. 7 shows a flow diagram of a method for monitoring propeller healthusing the arrangements of FIGS. 1, 2, 5 and 6, herein called “shaftmethod one”;

FIG. 8 shows a flow diagram of another method for monitoring propellerhealth using the arrangements of FIGS. 1, 2, 5 and 6, herein called“shaft method two”;

FIG. 9 shows schematically a third embodiment of first propeller healthmonitoring arrangement for a propeller having four blades;

FIG. 10 shows a cross section of the drive shaft of FIG. 9, taken alongthe line C-C, the drive shaft having four sensors, arranged in pairs ofdiametrically opposed sensors, mounted thereto;

FIG. 11 is a graph illustrating the shaft bending moments measured bytwo of the sensor pairs using the arrangement of FIGS. 9 and 10 for apropeller having four healthy (i.e. undamaged) blades;

FIG. 12 is a graph illustrating the shaft bending moments measured bytwo of the sensor pairs using the arrangement of FIGS. 9 and 10, for apropeller having four blades, of which one pair of diametrically opposedblades is healthy (undamaged) and at least one blade is damaged;

FIG. 13 shows schematically a fourth embodiment of a first propellerhealth monitoring arrangement for a propeller having three blades;

FIG. 14 shows a cross section of the drive shaft of FIG. 13, taken alongthe line D-D, the drive shaft having six sensors arranged in threediametrically opposed pairs mounted thereto;

FIG. 15 shows a flow diagram of a method for monitoring propeller healthusing the arrangements of FIGS. 9, 10, 13 and 14, herein called “shaftmethod three”;

FIG. 16 shows a flow diagram of another method for monitoring propellerhealth using the arrangements of FIGS. 9, 10, 13 and 14, herein called“shaft method four”;

FIG. 17 shows schematically a first embodiment of a second propellerhealth monitoring arrangement for a propeller having four blades;

FIG. 18 is a graph illustrating the cyclic strain gauge responsemeasured using the arrangement of FIG. 17, when all four blades arehealthy (i.e. undamaged);

FIG. 19 shows the graph of FIG. 18, on which there is superimposed anexemplary cyclic strain gauge response if one of the four blades isdamaged;

FIG. 20 shows schematically a second embodiment of a second propellerhealth monitoring arrangement for a propeller having three blades; and

FIG. 21 shows a flow diagram of a method for monitoring propeller healthusing the arrangements of FIGS. 17 and 20, herein called “hub method”.

DETAILED DESCRIPTION

The term “a first propeller health monitoring arrangement” is usedherein to describe an arrangement in which strain sensors are located onthe drive shaft of the propeller. First, second, third and fourthembodiments of such an arrangement are discussed below.

The term “a second propeller health monitoring arrangement” is usedherein to describe an arrangement in which strain sensors are located onthe hub arms of the propeller. First and second embodiments of such anarrangement are discussed below.

The terms “shaft method one”, “shaft method two”, “shaft method three”and “shaft method four” are used herein to describe four respectivemethods for monitoring propeller health utilising strain sensors mountedon the shaft of the propeller.

The term “hub method” is used herein to describe a method for monitoringpropeller health utilising strain sensors mounted on hub arms of thepropeller.

The term “propeller health monitoring” is a well known term in the artdescribing the monitoring of propellers to establish (i.e. detect ordetermine) whether damage has (or may have) occurred to propellers, inparticular aircraft propellers, and in particular aircraft propellerblades. In other words, to establish whether the health of the propellermay be, or is, impaired.

The inventors have recognised that all identical healthy blades on asingle propeller produce the same thrust as the propeller rotates andhave the same centrifugal force. As the propeller is rotated by thepropeller drive shaft, since a centreline of the propeller is inclinedat an angle relative to the direction of flight (or the oppositedirection defined by the free stream velocity vector), the drive shaftexperiences a bending moment comprising a sum of sinusoidal bendingmoments, i.e. a constant rotating bending moment. By way of explanation,every circumferential point on the shaft is exposed to the samesinusoidal variation in strain, thus there are an infinite number ofsinusoids. If the bending moment associated with each blade isconsidered (e.g. as determined from strain measurements measured bysensors aligned with the blades), there are N bending moment sinusoids,which are phase shifted by 2π/N where N is the number of blades on thepropeller.

The sinusoidal bending moments are cyclic bending moments whichfluctuate above and below a steady bending moment by equal amounts, i.e.they are centred around a steady bending moment. A steady bending momentof the drive shaft is the average bending moment experienced by thedrive shaft as it rotates about its axis. For a healthy, perfectlybalanced propeller, there is no steady bending on the shaft, i.e. thesteady bending moment is zero, and there is only an axial load (thrust).

The inventors have realised that these sinusoidal bending moments can bemeasured by fixing (i.e. mounting, attaching) strain sensorscorresponding to the blades around the shaft at positions on thecircumference of the shaft which are aligned with the circumferentialpositions at which the blades are located. Put another way, for apropeller having a plurality of blades extending radially outwardly fromhub arms of a propeller hub, which in turn extend radially outwardlyfrom a central axis extending through the propeller and a propellerdrive shaft, strain sensors corresponding to each of the blades can beprovided around a circumference of the drive shaft, each crossing aplane defined by the radial direction of the corresponding blade and thecentral axis, the plane being bounded by the central axis.

Thus, it will be appreciated that in embodiments of the first propellerhealth monitoring arrangement of the disclosure, where a sensor“corresponding” to a blade or “aligned with” a blade is referred to, itis intended to mean a sensor that is provided on the circumference ofthe drive shaft crossing a plane defined by the radial direction of saidblade and the central axis of the drive shaft, the plane being boundedby the central axis.

The sensors measure strain in the drive shaft, preferably continuouslyover time. The bending moments can be determined from the strainmeasurements made by the strain sensors, as will be discussed later.Thus, whilst the strain sensors measure strain, the strain sensor outputis directly related to bending moment. Consequently, in this disclosure,the strain sensors are sometimes referred to as monitoring or measuringbending moment. Further, it can be understood that a sensor has acorresponding bending moment, i.e. a bending moment determined from thestrain measured by that sensor. A sensor will also have a correspondingsteady bending moment calculated from measuring bending moment overtime.

The inventors have also realised that when a propeller blade has beendamaged, it may produce more or less thrust than a healthy propellerblade and may also experience a different centrifugal force compared toa healthy blade. Consequently, the damaged blade will give rise to asinusoidal bending moment centred around a non-zero steady bendingmoment.

The damaged blade will also absorb more or less power than the healthyopposing blade. This torque imbalance results in a steady shear force inthe propeller plane of rotation. The steady shear force results in asecond steady bending moment on the propeller shaft that is 90° awayfrom (i.e. 90° out of phase with) the steady bending moment produced bythe thrust imbalance. However, the magnitude of the moment produced bythe in-plane shear force, as determined from strain measurementsmeasured by the sensors on the shaft, is very small compared to themoment produced by the thrust imbalance. There are two main reasons forthe relatively small responses; one is that the magnitude of the shearforce is typically small. The second is that the distance between thesensors and the application point of the shear force (i.e. the propellerplane of rotation) is also small. These two factors combine to produce asmall bending moment at the sensors, which is generally not seen duringflight tests. Therefore, this second bending moment contribution is notconsidered in the present disclosure. For a propeller having four blades(and a sensor aligned with each blade), only the sensor corresponding tothe damaged blade or the sensor diametrically opposite the damaged bladewill monitor a change in steady bending moment (i.e. by virtue ofmeasuring strain from which bending moment is determined). In otherwords, if a sensor monitors a change in steady bending moment, theneither the blade aligned therewith or the blade diametrically opposedthereto is damaged. The sensor(s) corresponding to the healthy blade(s)will continue to monitor a zero steady bending moment since they are onthe neutral axis. The neutral axis is, by definition, the location on astructural member where the stress and strain produced by a bendingmoment is zero. For a circular shaft with a uniform cross-section, theneutral axis passes through the centre of the shaft and is aligned withthe bending moment vector.

For a propeller having an even number of blades greater than four, or anodd number of blades, if there is a damaged blade(s) all of the sensorswill monitor a deviation in steady bending moment from zero, even thosecorresponding to healthy blades. However, in the case of an odd numberof blades, the largest deviation will be monitored by the sensorcorresponding to the damaged blade. In the case of an even number ofblades greater than four, the largest deviation will be monitored by thesensor corresponding to or diametrically opposite to the damaged blade.

Additionally, the sinusoidal bending moment for a damaged blade may havea different amplitude compared to that of a healthy (i.e. undamaged)blade of the propeller.

Thus, by identifying a shift in steady bending moment away from a zerosteady bending moment and the magnitude of the shift, e.g. by comparingwith a threshold, it is possible to both establish if the health of thepropeller is impaired, and also identify which blade(s) are damaged.Damage detectable by identifying such a shift in bending moment mayinclude airfoil damage or oil in a blade cavity, which would create anaerodynamic or mass imbalance.

FIG. 1 shows a first embodiment of a first arrangement, in which apropeller 10 is attached to a propeller drive shaft 30. The propeller 10has blades 20 a, 20 b, 20 c and 20 d (blade 20 d is not shown) which arespaced equidistantly around the circumference of the propeller 10, andextend radially outwardly from hub arms 55 of a propeller hub 50. Thesehub arms 55, in turn, extend radially outwardly from a central axis, X,which extends through the propeller and propeller drive shaft.

Arranged around the drive shaft 30 are strain sensors 40 a, 40 b, 40 cand 40 d (sensor 40 d is not shown). Each sensor is alignedcircumferentially with one of the blades, such that the sensors 40 a, 40b, 40 c and 40 d are aligned respectively with blades 20 a, 20 b, 20 cand 20 d. Each strain sensor crosses a plane defined by the radialdirection of the corresponding blade and the central axis, the planebeing bounded by the central axis. The strain sensors measure strain(e.g. elongation or compression), and exemplary suitable strains sensorsare discussed later. However, it will be appreciated by the skilledperson that the type of measurements provided by the strain sensors maydepend on the type of strain sensor used. Typically, a strain sensor mayprovide a voltage output that is representative of the strain.Therefore, where “strain measurements” are discussed in relation to thepresent disclosure, this is intended to encompass such measurements thatare representative of the strain, e.g. measurements that are a functionof strain. Moreover, where “measuring the strain” is discussed inrelation to the present disclosure, this is intended to encompassobtaining measurements that are representative of strain.

This arrangement is exemplary only and other embodiments may have othernumbers of blades. There may be an even number of blades or odd numbersof blades; but in either case, at least some of the blades will eachhave a sensor aligned therewith. It is preferred the each blade isprovided with a corresponding sensor in order to provide the mostaccurate result, however in some embodiments sensors may only beprovided for some blades.

The cross section shown in FIG. 2 is taken along the line A-A in FIG. 1and shows sensors 40 a, 40 b, 40 c and 40 d as described above and asshown in FIG. 1. As described above, each of the sensors 40 a-d isaligned with a respective one of the blades 20 a-d.

In use, the drive shaft 30 of the propeller will rotate in the usualway, thereby rotating sensors 40 a, 40 b, 40 c and 40 d with blades 20a, 20 b, 20 c and 20 d. The sensors measure the strain (in this caseprovide a voltage that is representative of the strain, see discussionabove) in the shaft at each location. Strain is measured continuously orperiodically over time, preferably over multiple revolutions of thepropeller. This also applies to the later described embodiments.

The bending moment is determined from the measured strain (in this casefrom the voltage representative of the strain). It may be calculatedusing methods readily understood by those skilled in the art. Or, tosimplify matters, a calibration can be made between strain and bendingmoment, by finding a relationship between a known applied moment and themeasured strain. The response is typically very linear, so it is asimple constant to convert strain sensor output to bending moment. Inembodiments, this constant is input to the Data Acquisition System (DAS)computer together with the strain measurements so that the DAS computercan easily convert strain measurements to bending moment and provide abending moment output (i.e. in engineering units).

In either case, it will be appreciated that in embodiments of the firstpropeller health monitoring arrangement, the strain is measured by thestrain sensors and the strain sensor measurements are converted tobending moments by a processor, in an appropriate way.

In the present embodiment, this measured strain is input to a processor(not shown) for calculation of bending moment. The processor may belocated in the FADEC or in the nacelle, in which cases the measuredstrains may for example be transmitted via a slip, ring telemetry and/orWi-Fi from the rotating part to the static part and then to the FADEC ornacelle. This allows for real-time processing of the measured strains.If it is desirable to instead analyse data after a flight, it may alsobe possible to record and store data and download this at the end of theflight.

The combination of the sensors and the processor may together beconsidered as an apparatus or system.

The processor calculates the shaft bending moments from the measuredstrain values for each of the sensors 40 a, 40 b, 40 c and 40 d whichcorrespond to the bending moments in the plane of each respective blade20 a, 20 b, 20 c and 20 d. The bending moments calculated for eachsensor location are then analysed to determine if the blade associatedwith that sensor may be damaged, as is discussed further below. Sincestrain measurements are obtained over a period of time (and thus how thestrain changes over time is known), bending moment over a period of timecan be determined from these strain measurements, thus it is known howbending moment changes over time. This enables steady bending moment tobe determined as discussed later. This also applies to later describedembodiments of the first propeller health monitoring arrangement.

It should be noted that in this context, a bending moment of a shaft isa bending moment of the shaft in the frame of reference of the shaft,i.e. with respect to the shaft, so the frame of reference is taken asrotating with the propeller drive shaft. This is not the same as adynamic bending moment measured using a stationary object as thereference point and viewing the rotating propeller drive shaft as havinga relatively rotating bending moment.

FIG. 3 illustrates graphically the shaft bending moments over time, forthe propeller 10 of FIG. 1, calculated using strain data from two of thesensors: 40 a and 40 b. The skilled person would readily appreciate howto calculate bending moment from measured strain data. In this case, theblades 20 a, 20 b of the propeller 10 which correspond to these sensormeasurements are healthy.

The solid line 110 shows the bending moment of the drive shaft 30 overtime in a first plane defined as a plane having a normal being a vectorproduct of:

-   -   the diameter of the drive shaft 30 at the orientation at which        the first sensor 40 a, is connected thereto; and    -   the axis of rotation of the drive shaft 30.

In other words, the solid line 110 represents the bending momentcalculated from the first sensor.

The first plane rotates with the drive shaft, since the first sensor 40a rotates with the drive shaft.

The dotted line 120 shows the bending moment of the drive shaft 30 overtime in a second plane defined as a plane having a normal being a vectorproduct of:

-   -   the diameter of the drive shaft 30 at the orientation at which        the second sensor 40 b is connected thereto; and    -   the axis of rotation of the drive shaft 30.

In other words, the dotted line 120 represents the bending momentcalculated from the second sensor.

The second sensor 40 b is located at an orientation which is 90 degreesrotated about the axis of the drive shaft relative to the first sensor.The second plane rotates with the drive shaft 30, since the secondsensor 40 b rotates with the drive shaft.

Both of the bending moments 110, 120 are sinusoids centred around a zerosteady bending moment, shown by the dashed line 105. In other words,although the drive shaft may at times during rotation bend in onedirection in the respective plane and at other times during rotationbend in the opposite direction in the respective plane, the averagebending moment is zero. This means that the thrust produced by theblades 20 a, 20 b and the centrifugal force generated by the blades 20a, 20 b are equal, and indicates that each of blades 20 a, 20 b are (orare very likely to be) healthy, i.e. undamaged. Moreover, the bladediametrically opposite to each of blades 20 a, 20 b is also likely to behealthy.

Conversely, FIG. 4 illustrates graphically the shaft bending momentsover time, for the propeller 10 of FIG. 1, in the case that blade 20 aand/or the blade diametrically opposite to it, blade 20 c, is damaged.As shown in FIG. 4, although the dotted line 120 (representing thebending moment calculated using measurements by the second sensor 40 b)has not changed compared to FIG. 3 and is still centred about the zerosteady bending moment indicated by dashed line 105, the solid line 110(representing the bending moment calculated from the first sensor 40 a)has shifted to be centred about a non-zero steady bending moment, and isnow labelled as line 130.

So, the bending moment has an average bending moment which is offsetfrom zero bending moment. In other words, as the propeller drive shaft30 rotates, it tends to bend more in one direction than it does in theopposite direction. The non-zero average bending moment, i.e. the steadybending moment, is given by solid line 140 and the amount by which it isoffset from a zero average bending moment is indicated by arrow 150.

In this case, at least one of the blade 20 a corresponding to the firstsensor 40 a and the blade 20 c located diametrically opposite across thepropeller from the first sensor 40 a, has been damaged. (As mentionedearlier, in propellers with an even number of blades such that eachblade has a blade diametrically opposite thereof, a sensor aligned witha particular blade will monitor a deviation in steady bending moment ifeither that blade or its diametrically opposite counterpart is damaged).

Since this embodiment is for a four-bladed propeller, for all healthyblades which are not diametrically opposed across the propeller from adamaged blade, the measured steady bending moment will remain at zero.Thus the steady bending moment 105 for the cyclic bending moment givenby dotted line 120 (which corresponds to the measurement from the secondsensor 40 b located 90 degrees around the propeller from the firstsensor 40 a) does not deviate from zero in this case as thecorresponding blade 20 b (and the diametrically opposite blade 20 d) is(are) not damaged.

The four bladed propeller is unique because the sensor (e.g. 40 b)aligned with one undamaged blade (e.g. 20 b) and the sensor (e.g. 40 d)aligned with its opposing undamaged counterpart (e.g. 20 d) are on theneutral axis for a moment caused by damage to either (or both) of theother two blades (40 a, 40 c). Hence, the steady bending momentdetermined from measurements by sensors aligned with the undamagedblades does not deviate from zero. If, on the other hand, there are moreblades with sensors, they will not be on the neutral axis and willrespond to damage to the other blades. Thus, in other propellerembodiments where there are an even number of blades greater than four,or an odd numbers of blades, as mentioned above even healthy blades willhave a steady bending moment deviated from zero. But, the magnitude ofthe deviation will be less than the deviation for damaged blades.

It is the magnitude of the deviation from the zero bending moment foreach sensor which is determined by the processor in order to establishif a blade (or, for propellers with even numbers of blades, the bladediametrically opposed to it) is damaged, and thus whether the health ofthe propeller is impaired.

Depending on what damage has occurred, there may be a positive ornegative steady bending moment, i.e. a positive or negative offset 150from zero, although the latter is not shown in FIG. 4. The quantity tobe evaluated is therefore the magnitude of the offset 150. If themagnitude of the offset 150 exceeds a predetermined threshold, it isestablished that either the blade corresponding to the sensor yieldingthe offset steady bending moment 150 exceeding the threshold, or theblade diametrically opposed from that blade, is damaged. Consequently,an alert for maintenance can be triggered and the identified blades ofthe propeller can be inspected for damage and repair or replacement workcan be carried out. If the magnitude of the steady bending moment offsetis below the predetermined threshold, it is established that the offsetis not significant enough to be indicative of a damaged blade.

The threshold for the magnitude of the offset is specific to theparticular propeller and may depend on various factors such as number ofblades, total thrust and blade diameter, all of which can affect theobserved steady bending moments. For example, from a thrust-standpoint,even brand new blades which are intended to be identical may not beperfectly balanced (i.e. they may have inherent small thrustdifferences), due to manufacturing tolerances. Furthermore, there may bea bias in the observed steady bending moments due to a drift on thestrain sensors because of temperature compensation. In order that sucheffects are not accidentally confused with steady bending moment offsetsdue to blade damage, it should be established empirically what is theextent of any offset present when the blades are healthy. A suitableoffset threshold for blade damage can then be chosen which would clearlyindicate a blade being damaged.

The skilled person would readily understand that an appropriatethreshold can be established empirically from test data, e.g. dataobtained during a flight test. This is well within the capability of theskilled person.

The above described method utilises a comparison with a threshold of theoffset between a zero bending moment and the average bending moment, inorder to detect a damaged blade (this is described in more detail lateras the “shaft method one”, with reference to FIG. 7).

However, in another method (described in more detail later as the “shaftmethod two”, with reference to FIG. 8), the average, i.e. steady,bending moment corresponding to each sensor may be compared with that ofthe other sensors in order to detect the presence of a damaged blade.This could be achieved for example by calculating the cumulative “error”in the steady bending moments measured by each strain gauge as given bythe following formula:

${{error}\mspace{14mu}{SBM}_{n}} = \left\lbrack {\sum\limits_{i = 1}^{N}\;\left( {{SBM}_{n} - {SBM}_{i}} \right)^{2^{2}}} \right\rbrack^{0.5}$

where n is the reference number for the strain sensor in question and Nis the total number of strain sensors.

From this, it can be established that the blade corresponding to thestrain sensor which gives rise to the largest “error” in the steadybending moment (or, for even numbers of blades, the blade diametricallyopposite) may be damaged.

A second embodiment of the first propeller monitoring arrangement, inwhich a propeller has an odd number of blades, is now described. FIG. 5shows a propeller 210 having three blades 220 a, 220 b (not shown) and220 c. The blades 220 a, 220 b, 220 c are connected to the hub 250 ofthe propeller via hub arms 255. Attached to the propeller is a driveshaft 230 having three sensors 240 a, 240 b, 240 c, the arrangement ofwhich can be seen more clearly in FIG. 6.

FIG. 6 shows a cross section through the drive shaft along the line B-Bin FIG. 5. The circumferential locations of each of the blades 220 a,220 b and 220 c are indicated by radial lines. There are three sensors:sensor 240 a is aligned with blade 220 a; sensor 240 b is aligned withblade 220 b; and sensor 240 c is aligned with blade 220 c.

The bending moments determined from strain measurements measured by thestrain sensors 240 a, 240 b, 240 c when plotted graphically wouldresemble those shown in FIG. 3 for a healthy propeller having an evennumber of blades. That is, the sinusoidal bending moments would becentred around an average bending moment of zero, i.e. have a zerosteady bending moment. However, if a single blade were to becomedamaged, the steady bending moments for all of the blades would beoffset from zero, while the offset of the steady bending momentdetermined from the strain measurement measured by the sensorcorresponding to the damaged blade would have the greatest magnitude.

The two methods (shaft method one and shaft method two) for monitoringthe health of a four-bladed propeller, as described above, can also beused with this second embodiment of the disclosure.

In the first method (shaft method one), the magnitude of the steadybending moments can be compared against a threshold. Since the magnitudeof the offset steady bending moment corresponding to the damaged bladewill be the largest, the threshold can be selected, for example in themanner described above, such that only significantly large offsets willexceed the threshold. Thus the damaged blade can be identified.Alternatively, the amount by which the steady bending moments exceed thethreshold can be calculated. The greatest amount is indicative of whichblade is damaged.

In the second method (shaft method two), the steady bending momentcorresponding to sensor may be compared with that of the other sensorsin order to detect the presence of a damaged blade. This could beachieved for example by calculating the cumulative “error” in the steadybending moments determined from the strain measured by each sensor asgiven by the following formula:

${{error}\mspace{14mu}{SBM}_{n}} = \left\lbrack {\sum\limits_{i = 1}^{N}\;\left( {{SBM}_{n} - {SBM}_{i}} \right)^{2^{2}}} \right\rbrack^{0.5}$

where n is the reference number for the strain sensor in question and Nis the total number of strain sensors.

From this, it can be established that the strain sensor which gives riseto the largest “error” in the steady bending moment has a correspondingblade which may be damaged.

FIG. 7 describes in more detail the “shaft method one” 300 for apropeller with an even or an odd number of blades. At step 310, strainsensors are installed on the drive shaft of the propeller, onecorresponding to each blade. Each sensor provides a measurement of thestrain in the drive shaft at that location.

At step 320, the strain sensor data is analysed to determine the steadybending moment corresponding to each sensor. This includes using aprocessor to convert the strain measurements to bending moments andperform various algorithms to find the steady bending moment. A firstalgorithm calculates the time taken for one revolution of the propeller,given by (RPM/60)^−1. A second algorithm records the maximum and minimumbending moment measured by each sensor in each revolution. A thirdalgorithm calculates the steady bending moment (SBM) for each sensor ineach revolution by taking the average of the recorded maximum andminimum bending moments, i.e. (max+min)/2.

At step 330, the magnitudes of the calculated steady bending moments(SBM) corresponding to each sensor are compared to a threshold. Adecision is taken as to whether the steady bending moment magnitudesexceed the threshold. If none of them exceed the threshold, then at step340, the blades are deemed healthy and the method returns to step 320.Otherwise, if one or more steady bending moment magnitude does exceedthe threshold, then the method proceeds with step 350.

At step 350, the method determines if the propeller has an odd number ofblades. If this is the case, then it is established at step 360 that theblade aligned with the sensor, the strain measurement of which yieldedthe largest steady bending moment exceeding the threshold, is damaged.Otherwise, if there is an even number of blades, then it is establishedat step 370 that the blade aligned with the sensor and/or the bladediametrically opposite to the sensor, the strain measurement of whichyielded the largest steady bending moment exceeding the threshold, isdamaged.

An alert (e.g. a visual or aural indicator) can then be raised and theidentified blade or pair of blades can then be inspected formaintenance.

FIG. 8 describes in more detail the “shaft method two” 400 for apropeller with an even or an odd number of blades. At step 410, a strainsensor (e.g. a full bridge strain gauge) is installed on the drive shaftof the propeller corresponding to each blade. Each sensor measures thestrain in the drive shaft at that location.

At step 420, the strain sensor data is analysed to determine the steadybending moment corresponding to each sensor. This includes using aprocessor to convert the strain measurements to bending moments andperform various algorithms. A first algorithm calculates the time takenfor one revolution of the propeller, given by (RPM/60)^−1. A secondalgorithm records the maximum and minimum bending moment measured byeach sensor in each revolution. A third algorithm calculates the steadybending moment (SBM) for each sensor in each revolution by taking theaverage of the recorded maximum and minimum bending moments, i.e.(max+min)/2).

At step 430, the magnitudes of the calculated steady bending moments(SBM) corresponding to each sensor are compared to each other. Forexample, the cumulative “error” may be calculated in the steady bendingmoments corresponding to each strain sensor as given by the followingformula:

${{error}\mspace{14mu}{SBM}_{n}} = \left\lbrack {\sum\limits_{i = 1}^{N}\;\left( {{SBM}_{n} - {SBM}_{i}} \right)^{2^{2}}} \right\rbrack^{0.5}$

where n is the reference number for the strain sensor in question and Nis the total number of strain sensors. From this, it can be establishedthat the strain sensor which gives rise to the largest “error” in thesteady bending moment (i.e. an error outside a defined tolerance) has acorresponding blade (or blades) which may be damaged. A decision is thentaken as to whether the steady bending moments are equal within adefined tolerance. Thus if all of the “error”s calculated above arewithin a defined tolerance, such as for example, a 3%, 5%, 10%, 15% or20% tolerance, the blades are deemed at step 440 to be healthy and themethod returns to step 420. Otherwise, if any of the “error”s calculatedabove are not within a defined tolerance, such as for example, a 3%, 5%,10%, 15% or 20% tolerance, then the method proceeds with step 450.

At step 450, the method determines if the propeller has an odd number ofblades. If this is the case, then it is established at step 460 that theblade aligned with the sensor, the strain measurement of which hasyielded the largest “error” compared to the “error” of the othersensors, or which has yielded the largest difference in steady bendingmoment compared to the steady bending moments of the other sensors, isdamaged. Otherwise, if there is an even number of blades, then it isestablished at step 470 that the blade aligned with the sensor and/orthe blade diametrically opposite to the sensor, the strain measurementof which has yielded the largest “error” compared to the “error” of theother sensors, or which has yielded the largest difference in steadybending moment compared to the steady bending moments of the othersensors, is damaged.

An alert (e.g. a visual or aural indicator) can then be raised and theidentified blade or pair of blades can then be inspected formaintenance.

In the first and second embodiments of the first propeller healthmonitoring arrangement described above, a sensor is provided for each ofat least some of the blades of the propeller. Strain in the propellerdrive shaft is measured using these multiple sensors (which may bedenoted “primary sensors”), with each sensor providing a respectivestrain measurement. These “primary sensors” are located around acircumference of the drive shaft, and each primary sensor is locatedsuch that it crosses a plane defined by the radial direction of a bladeand the central axis, the plane being bounded by the central axis. Thus,the strain measured by each strain sensor is associated with aparticular blade or blade pair comprising the particular blade and adiametrically opposed blade.

However, the present inventors have discovered that advantages areoffered by using pairs of strain sensors, with a pair of strain sensorsbeing associated with a particular blade or blade pair. Each pair ofstrain sensors can comprise a “primary sensor”, and a “secondarysensor”, wherein the secondary sensor is located around thecircumference of the drive shaft diametrically opposite to a primarysensor. The terminology “primary” and “secondary” is merely used todistinguish between the sensors, and does not infer that one ispreferable in any way to the other. In embodiments utilising such a pairof strain sensors, the method of the disclosure additionally comprisesmeasuring strain in the propeller drive shaft using the secondarysensors, and the steady bending moment associated with each blade iscalculated using the strain data obtained by both sensors of each pair.This offers the advantage that axial load can be cancelled out.Embodiments utilising pairs of strain sensors are now described.

Sinusoidal bending moments can be measured by fixing strain sensors inpairs corresponding to the blades around the shaft, each sensor paircomprising a sensor at a position on the circumference of the shaftwhich is aligned with the circumferential position at which a blade islocated (denoted a primary sensor) and a sensor at a positiondiametrically opposite from it across the shaft (denoted a secondarysensor), on the circumference of the shaft. Put another way, for apropeller having a plurality of blades extending radially outwardly fromhub arms of a propeller hub, which in turn extend radially outwardlyfrom a central axis extending through the propeller and a propellerdrive shaft, diametrically opposed strain sensor pairs corresponding toeach of the blades can be provided around a circumference of the driveshaft, each strain sensor pair comprising two sensors crossing a planedefined by the radial direction of the corresponding blade and thecentral axis and being diametrically opposed to one another across thedrive shaft.

For a propeller having an odd number of blades N, there will be N sensorpairs, while for a propeller having an even number of blades N, therewill be N/2 sensor pairs, since each pair of diametrically opposedblades shares a sensor pair. It will be appreciated that for an evennumber of blades, if a particular blade Y has a primary sensor alignedtherewith and a secondary sensor diametrically opposite, that secondarysensor is also aligned with a blade, call this blade Z. The secondarysensor for blade Y is then the primary sensor for blade Z, whilst theprimary sensor for blade Z is the secondary sensor for blade Y. Inpractice, therefore, each pair of diametrically opposed blades shares asensor pair and shares the sensor measurements made by the pair. Thus,the strain measurements for blade Y will be the same as the strainmeasurements for blade Z.

When a propeller blade has been damaged, as discussed previously, it mayproduce more or less thrust than a healthy propeller blade and may alsoexperience a different centrifugal force compared to a healthy blade.Consequently, the damaged blade will give rise to a sinusoidal bendingmoment centred around a non-zero steady bending moment.

For a propeller having four blades (and thus two sensor pairs), oneblade (or two opposing blades) of which are damaged, only the sensorpair corresponding to the damaged blade(s) will monitor a change insteady bending moment. The sensor pair corresponding to the healthyblades will continue to monitor a zero steady bending moment since theyare on the neutral axis. As mentioned above, the neutral axis is, bydefinition, the location on a structural member where the stress andstrain produced by a bending moment is zero. For a circular shaft with auniform cross-section, the neutral axis passes through the centre of theshaft and is aligned with the bending moment vector.

For a propeller having an even number of blades greater than four, or anodd number of blades, if there is a damaged blade(s) all of the sensorpairs will monitor a deviation in steady bending moment from zero, eventhose corresponding to healthy blades. However, the largest deviationwill be monitored by the sensor pair corresponding to the damaged blade.

Additionally, the sinusoidal bending moment for a damaged blade orblades may have a different amplitude compared to that of a healthy(i.e. undamaged) blade or blades of the propeller.

Thus, by identifying a shift in steady bending moment away from a zerosteady bending moment and the magnitude of the shift, e.g. by comparingwith a threshold, it is possible to both establish if the health of thepropeller is impaired, and also identify which blade(s) are damaged.Damage detectable by identifying such a shift in bending moment mayinclude airfoil damage or oil in a blade cavity, which would create anaerodynamic or mass imbalance.

FIG. 9 shows a third embodiment of a first arrangement, in which apropeller 110 is attached to a propeller drive shaft 130. The propeller110 has four blades 120 a, 120 b, 120 c and 120 d (blade 120 d is notshown) which are spaced equidistantly around the circumference of thepropeller 110, and extend radially outwardly from hub arms 155 of apropeller hub 150. These hub arms 155, in turn, extend radiallyoutwardly from a central axis X, which extends through the propeller 110and propeller drive shaft 130.

Arranged around the drive shaft 130 are strain sensors 140 a, 140 b, 140c, 140 d (sensor 140 d is not shown), each corresponding to one of theblades 120 a, 120 b, 120 c and 120 d respectively.

Sensors 140 a and 140 c form primary and secondary sensors respectivelyof a sensor pair 140′, with the secondary sensor 140 c being arrangeddiametrically opposite across the drive shaft 130 from the primarysensor 140 a. Sensors 140 b and 140 d form primary and secondary sensorsrespectively of another sensor pair 140″, with the secondary sensor 140d being arranged diametrically opposite across the drive shaft 130 fromthe primary sensor 140 b. Each blade is thus provided with a sensorpair: blades 120 a and 120 c are provided with sensor pair 140′, whilstblades 120 b and 120 d are provided with sensor pair 140″.

The primary strain sensor of each pair 140′, 140″, crosses a planedefined by the radial direction of the blade for which the sensor pairis provided and the central axis, the plane being bounded by the centralaxis.

As described previously, since there are an even number of bladesequidistantly spaced around the circumference of the propeller, theprimary sensor of a sensor pair corresponding to one blade also acts asa secondary sensor of a sensor pair corresponding to the diametricallyopposed blade. Thus for a propeller having an even number of blades N,there are N sensors, such that two diametrically opposed blades share asensor pair. In other words, opposing blades “share” the same sensors.Each strain sensor in each strain sensor pair crosses a plane defined bythe radial direction of the corresponding blade and the central axis.

Exemplary suitable strain sensors are discussed later. The two strainsensors in each pair will generally be wired together such that twicethe strain output is provided for a given shaft bending moment, thusyielding greater accuracy.

The arrangement of FIGS. 9 and 10 is exemplary only and otherembodiments may have other numbers of blades. There may be an evennumber of blades or odd numbers of blades, the latter being describedbelow in relation to FIG. 13. In either case, there will be a strainsensor pair, or “bending pair” provided for each of at least some of theblades, with one sensor of the pair being aligned with the blade andanother sensor being arranged diametrically opposite.

The cross section shown in FIG. 10 is taken along the line C-C in FIG. 9and shows sensors 140 a, 140 b, 140 c and 140 d as described above andas shown in FIG. 9. As described above, each of the sensor pairs 140′(140 a and 140 c), 140″ (140 b and 140 d) is aligned with two of theblades 120 a, 120 c, 120 b, 120 d, such that two diametrically opposedblades share a sensor pair.

In use, the drive shaft 130 of the propeller will rotate in the usualway, thereby rotating the sensor pairs 140′, 140″ with the blades. Thepairs of strain sensors 140 a and 140 c, 140 b and 140 d measure thestrain in the drive shaft 130 at each location. This measured strain isinput to a processor (not shown) for calculation of bending moments asdescribed below. The processor may be located in the FADEC or in thenacelle, in which cases the measured strains may for example betransmitted via a slip ring, telemetry and/or Wi-Fi from the rotatingpart to the static part and then to the FADEC or nacelle. This allowsfor real-time processing of the measured strains. If it is desirable toinstead analyse data after a flight, it may also be possible to recordand store data and download this at the end of the flight.

The combination of the sensors and the processor may together beconsidered as an apparatus or system.

The processor calculates the shaft bending moments from the measuredstrain values for each of the sensor pairs 140′, 140″ which correspondto the bending moments in the plane of each blade 120 a and 120 c, 120 band 120 d. The bending moments calculated for each sensor pair locationare then analysed to determine if the blade or blades associated withthat sensor pair may be damaged, as is discussed further below.

It should be noted that in this context, a bending moment of a shaft isa bending moment of the shaft in the frame of reference of the shaft,i.e. with respect to the shaft, so the frame of reference is taken asrotating with the propeller drive shaft. This is not the same as adynamic bending moment measured using a stationary object as thereference point and viewing the rotating propeller drive shaft as havinga relatively rotating bending moment.

FIG. 11 illustrates graphically the shaft bending moments over time, forthe propeller 110 of FIG. 9, calculated using strain data from both ofthe sensor pairs. The skilled person would readily appreciate how tocalculate bending moment from measured strain data. In this case, theblades of the propeller 110 which correspond to these sensor pairmeasurements are healthy.

The solid line 1110 shows the bending moment of the drive shaft 130 overtime in a first plane defined as a plane having a normal being a vectorproduct of:

-   -   the diameter of the drive shaft 130 at the orientation at which        the sensor pair 140′ is connected thereto; and    -   the axis of rotation of the drive shaft 130.

The first plane rotates with the drive shaft, since the sensor pair 140′rotates with the drive shaft 130.

The dotted line 1120 shows the bending moment of the drive shaft 130over time in a second plane defined as a plane having a normal being avector product of:

-   -   the diameter of the drive shaft 130 at the orientation at which        the sensor pair 140″ is connected thereto; and    -   the axis of rotation of the drive shaft 130.

The sensor pair 140″ is located at an orientation which is 90 degreesrotated about the axis of the drive shaft relative to the sensor pair140′. The second plane rotates with the drive shaft 130, since thesensor pair 140″ rotates with the drive shaft.

Both of the bending moments 1110, 1120 are sinusoids centred around azero steady bending moment, shown by the dashed line 1105. In otherwords, although the drive shaft may at times during rotation bend in onedirection in the respective plane and at other times during rotationbend in the opposite direction in the respective plane, the averagebending moment is zero. This means that the thrust produced by theblades and the centrifugal force generated by the blades are equal, andis indicative of healthy propeller blades and thus a healthy propeller.

Conversely, FIG. 12 illustrates graphically the shaft bending momentsover time, for the propeller 110 of FIG. 9, in the case that at leastone of the blades in the pair of blades corresponding to sensor pair140′ is damaged. As shown in FIG. 12, although the dotted line 1120(representing the bending moment calculated using strain measurementsfrom sensor pair 140″) has not changed compared to FIG. 11 and is stillcentred about the zero bending moment indicated by dashed line 1105, thesolid line 1110 (representing the bending moment calculated using strainmeasurements from sensor pair 140′) has shifted to be centred about anon-zero steady bending moment, and is now labelled as line 1130. Inthis case, as the propeller has an even number of blades and each sensorpair corresponds to two blades, then one of the two blades correspondingto the sensor pair 140′ has been damaged and so the bending moment hasan average bending moment which is offset from zero bending moment. Inother words, as the propeller drive shaft 130 rotates, it tends to bendmore in one direction than it does in the opposite direction. Thenon-zero average bending moment, i.e. the steady bending moment, isgiven by solid line 1140 and the amount by which it is offset from azero average bending moment 1105 is indicated by arrow 1150.

Since this embodiment is a four-bladed propeller, for all healthy bladeswhich are not diametrically opposed across the propeller from a damagedblade, the measured steady bending moment will remain at zero. Thus thesteady bending moment 1105 (i.e. the average bending moment) for thecyclic bending moment given by dotted line 1120, which corresponds tothe measurement from the sensor pair 140″ does not deviate from zero inthis case as the corresponding blades are not damaged. The four bladedpropeller is unique because the sensor pair (e.g. 140″) aligned with anundamaged blade pair (140 b, 140 d) is on the neutral axis for a momentcaused by damage to either (or both) of the other two blades (140 a, 140c). Hence, the steady bending moment determined from measurements by thesensor pair aligned with the undamaged blades does not deviate fromzero.

If, on the other hand, there are more blades with sensors, they will notbe on the neutral axis and will respond to damage to the other blades.Thus, in other propeller embodiments where there are even numbers ofblades greater than four or odd numbers of blades, as mentionedpreviously, even healthy blades will have a steady bending momentdeviated from zero, but the magnitude will be less than the deviationfor damaged blades.

It is the magnitude of the deviation from the zero bending moment foreach sensor pair which is determined by the processor in order toestablish if a blade or blades corresponding to the respective sensorpair is/are damaged, and thus whether the health of the propeller isimpaired.

Depending on what damage has occurred, there may be a positive ornegative steady bending moment, i.e. a positive or negative offset 1150from zero, although the latter is not shown in FIG. 12. The quantity tobe evaluated is therefore the magnitude of the offset 1150. If themagnitude of the offset 1150 exceeds a predetermined threshold, it isestablished that at least one of the blades corresponding to the sensorpair yielding the offset steady bending moment 1150 exceeding thethreshold, is damaged.

Consequently, an alert for maintenance can be triggered and theidentified blades of the propeller can be inspected for damage andrepair or replacement work can be carried out. If the magnitude of thesteady bending moment offset is below the predetermined threshold, it isestablished that the offset is not significant enough to be indicativeof a damaged blade.

The threshold for the magnitude of the offset is specific to theparticular propeller and may depend on various factors such as thenumber of blades, total thrust and blade diameter, both of which canaffect the observed steady bending moments. For example, from athrust-standpoint, even brand new blades which are intended to beidentical may not be perfectly balanced (i.e. they may have inherentsmall thrust differences), due to manufacturing tolerances. Furthermore,there may be a bias in the observed steady bending moments due to adrift on the strain sensors because of temperature compensation. Inorder that such effects are not accidentally confused with steadybending moment offsets due to blade damage, it should be establishedempirically what is the extent of any offset present when the blades arehealthy. A suitable offset threshold for blade damage can then be chosenwhich would clearly indicate a blade being damaged.

The skilled person would readily understand that an appropriatethreshold can be established empirically from test data, e.g. dataobtained during a flight test. This is well within the capability of theskilled person.

The above described method utilises a comparison with a threshold of theoffset between a zero bending moment and the average bending moment, inorder to detect a damaged blade (this is described in more detail lateras “shaft method three”, with reference to FIG. 15). However, in anothermethod (described in more detail later as the “shaft method four”, withreference to FIG. 16), the average, i.e. steady, bending momentcorresponding to each sensor pair may be compared with that of the othersensor pair(s) in order to detect the presence of a damaged blade. Thiscould be achieved for example by calculating the cumulative “error” inthe steady bending moments determined from measurements by each pair ofstrain gauges as given by the following formula:

${{error}\mspace{14mu}{SBM}_{n}} = \left\lbrack {\sum\limits_{i = 1}^{N}\;\left( {{SBM}_{n} - {SBM}_{i}} \right)^{2^{2}}} \right\rbrack^{0.5}$

where n is the reference number for the strain gauge sensor pair inquestion and N is the total number of strain gauge sensor pairs.

From this, it can be established that the pair of strain gauges whichgives rise to the largest “error” in the steady bending moment have acorresponding blade or blades which may be damaged.

A fourth embodiment of the first propeller health monitoring arrangementin which a propeller with an odd number of blades is provided withsensor pairs is now described. FIG. 13 shows a propeller 1210 havingthree blades 1220 a, 1220 b (not shown) and 1220 c. The blades 1220 a,1220 b, 1220 c are connected to the hub 1250 of the propeller 1210 viahub arms 1255. Attached to the propeller 1210 is a drive shaft 1230having six sensors 1240 a, 1240 b, 1240 c, 1240 d, 1240 e, 1240 f, thearrangement of which can be seen more clearly in FIG. 14.

FIG. 14 shows a cross section through the drive shaft along the line D-Din FIG. 13. The circumferential locations of each of the blades 1220 a,1220 b and 1220 c are indicated by radial lines. There are three pairsof sensors. Sensors 1240 a and 1240 d are aligned with blade 1220 a(sensor pair 1240′). Sensors 1240 b and 1240 e are aligned with blade1220 b (sensor pair 1240″). Sensors 1240 c and 1240 f are aligned withblade 1220 c (sensor pair 1240′″). Or, it may be considered that sensor1240 a is a primary sensor for blade 1220 a and is aligned therewith,with sensor 1240 d forming a diametrically opposed secondary sensor.Sensor 1240 b is a primary sensor for blade 1220 b and is alignedtherewith, with sensor 1220 e forming a diametrically opposed secondarysensor. Sensor 1240 c is a primary sensor for blade 1220 c and isaligned therewith, with sensor 1220 f forming a diametrically opposedsecondary sensor. Thus for a propeller having an odd number N of blades,there will be N pairs of sensors, i.e. 2N sensors in total. In thiscase, no two blades share a sensor pair.

The bending moments determined from strain measurements made by thestrain sensors 1240 a-f when plotted graphically would resemble thoseshown in FIG. 11 for a healthy propeller having an even number ofblades. That is, the sinusoidal bending moments would be centred aroundan average bending moment of zero, i.e. have a zero steady bendingmoment. However, if a blade was to become damaged, the steady bendingmoments for all of the blades would be offset from zero, while theoffset of the steady bending moment determined from strain measurementsmade by the sensor pair corresponding to the damaged blade would havethe greatest magnitude.

Shaft method three and shaft method four for monitoring the health of afour-bladed propeller, as described above, can also be used with thisfourth embodiment of the disclosure.

In shaft method three, the magnitude of the offset steady bendingmoments can be compared against a threshold. Since the magnitude of theoffset steady bending moment corresponding to the damaged blade will bethe largest, the threshold can be selected such that only significantlylarge offsets will exceed the threshold. Thus the damaged blade can beidentified. Alternatively, the greatest steady bending moment offset maybe considered indicative of a blade which is damaged.

In shaft method four, the steady bending moment corresponding to eachsensor pair may be compared with that of the other sensor pairs in orderto detect the presence of a damaged blade. This could be achieved forexample by calculating the cumulative “error” in the steady bendingmoments corresponding to each pair of strain gauges as given by thefollowing formula:

${{error}\mspace{14mu}{SBM}_{n}} = \left\lbrack {\sum\limits_{i = 1}^{N}\;\left( {{SBM}_{n} - {SBM}_{i}} \right)^{2^{2}}} \right\rbrack^{0.5}$

where n is the reference number for the strain gauge sensor pair inquestion and N is the total number of strain gauge sensor pairs.

From this, it can be established that the pair of strain gauges whichgives rise to the largest “error” in the steady bending moment has acorresponding blade which may be damaged.

FIG. 15 describes in more detail the “shaft method three” 1300 for apropeller with an even or an odd number of blades. At step 1310, a pairof strain gauge sensors is installed on the drive shaft of the propellercorresponding to each blade. In each sensor pair, one sensor iscircumferentially aligned with the blade, while the other is located ata diametrically opposite position on the drive shaft. For an N-bladedpropeller, if N is even, there will be N/2 sensor pairs, sincediametrically opposed blades will share a sensor pair. If N is odd,there will be N sensor pairs. Each sensor pair is set up to measure abending moment (BM).

At step 1320, the strain sensor data is analysed to determine the steadybending moment corresponding to each sensor. This includes using aprocessor to convert the strain measurements to bending moments andperform various algorithms. A first algorithm calculates the time takenfor one revolution of the propeller, given by (RPM/60)^−1. A secondalgorithm records the maximum and minimum bending moment measured byeach sensor pair in each revolution. A third algorithm calculates thesteady bending moment (SBM) for each sensor pair in each revolution bytaking the average of the recorded maximum and minimum bending moments,i.e. (max+min)/2.

At step 1330, the magnitudes of the calculated steady bending moments(SBM) corresponding to each sensor pair are compared to a threshold. Adecision is taken as to whether the steady bending moment magnitudesexceed the threshold. If this not the case, then at step 1340, theblades are deemed healthy and the method returns to step 1320.Otherwise, if the steady bending moment magnitudes do exceed thethreshold, then the method proceeds with step 1350.

At step 1350, the method determines if the propeller has an odd numberof blades. If this is the case, then it is established at step 1360 thatthe blade aligned with the sensor pair yielding the largest steadybending moment exceeding the threshold is damaged. Otherwise, if thereis an even number of blades, then it is established at step 1370 thatthe blade aligned with the primary sensor of the sensor pair (and/or theblade diametrically opposite) yielding the largest steady bending momentexceeding the threshold is damaged.

The identified blade or pair of blades can then be inspected formaintenance at a later point and an alert for this can be raised.

FIG. 16 describes in more detail “shaft method four” 1400 for apropeller with an even or an odd number of blades. At step 1410, a pairof strain gauge sensors (e.g. full bridge strain gauges) is installed onthe drive shaft of the propeller corresponding to each blade. In eachsensor pair, one sensor is circumferentially aligned with the blade,while the other is located at a diametrically opposite position on thedrive shaft. For an N-bladed propeller, if N is even, there will be N/2sensor pairs, since diametrically opposed blades will share a sensorpair. If N is odd, there will be N sensor pairs. Each sensor pair is setup to measure a bending moment (BM).

At step 1420, the strain sensor data is analysed to determine the steadybending moment corresponding to each sensor. This includes using aprocessor to convert the strain measurements to bending moments andperform various algorithms. A first algorithm calculates the time takenfor one revolution of the propeller, given by (RPM/60)^−1. A secondalgorithm records the maximum and minimum bending moment measured byeach sensor pair in each revolution, i.e. (max+min)/2. A third algorithmcalculates the steady bending moment (SBM) for each sensor pair in eachrevolution by taking the average of the recorded maximum and minimumbending moments.

At step 1430, the magnitudes of the calculated steady bending moments(SBM) corresponding to each sensor pair are compared to each other. Forexample, the cumulative “error” in the steady bending momentscorresponding to each pair of strain gauges may be calculated as givenby the following formula:

${{error}\mspace{14mu}{SBM}_{n}} = \left\lbrack {\sum\limits_{i = 1}^{N}\;\left( {{SBM}_{n} - {SBM}_{i}} \right)^{2^{2}}} \right\rbrack^{0.5}$

where n is the reference number for the strain gauge sensor pair inquestion and N is the total number of strain gauge sensor pairs. Fromthis, it can be established that the pair of strain gauges which giverise to the largest “error” in the steady bending moment has acorresponding blade (or blades) which may be damaged. It is then decidedas to whether the steady bending moments are equal within a definedtolerance. Thus if all of the “error”s calculated above are within adefined tolerance, such as for example, a 3%, 5%, 10%, 15% or 20%tolerance, the blades are deemed at step 1440 to be healthy and themethod returns to step 1420. Otherwise, if all of the “error”scalculated above are not within a defined tolerance, such as forexample, a 3%, 5%, 10%, 15% or 20% tolerance, then the method proceedswith step 1450.

At step 1450, the method determines if the propeller has an odd numberof blades. If this is the case, then it is established at step 1460 thatthe blade aligned with the sensor pair which yields the largest “error”compared to the “error” of the other sensor pairs, or which has thelargest difference in steady bending moment compared to the steadybending moments yielded by other pairs, is damaged. Otherwise, if thereis an even number of blades, then it is established at step 1470 that atleast one of the blades aligned with the sensor pair which yields thelargest “error” compared to the “error” yielded by the other sensorpairs, or which has the largest difference in steady bending momentcompared to the steady bending moments yielded by the other pairs, isdamaged.

An alert (e.g. a visual or aural indicator) can then be raised and theidentified blade or pair of blades can then be inspected formaintenance.

The above embodiments relate to a first propeller health monitoringarrangement in which strain sensors are located on the propeller driveshaft. However, a second propeller health monitoring arrangement is alsoprovided, in which strain sensors are located on hub arms of thepropeller. Embodiments of such a second arrangement are now described.

The inventors have realised that damage to blades of a propeller canalso be detected by, in a second arrangement, placing strain sensors onhub arms of a propeller, with each strain sensor corresponding to apropeller blade. Each strain sensor must be circumferentially alignedwith the propeller blade, e.g. offset from the propeller blade along aline parallel to the rotational axis of the propeller (axially offset).Each strain sensor should also be located radially inward of thepropeller blade with which it is associated, and along a radial lineextending from a central axis of the propeller hub along the blade.Thus, for a propeller having a plurality of blades extending radiallyoutwardly from hub arms of a propeller hub, which in turn extendradially outwardly from a central axis extending through the propellerand a propeller drive shaft, strain sensors can be providedcorresponding to each of the blades, preferably on an axially forwardside of the hub arm of the corresponding blade, each crossing a planedefined by the radial direction of the corresponding blade and thecentral axis, the plane being bounded by the central axis.

The strain sensors measure strain over time, e.g., continuously orperiodically. The cyclic response of each strain sensor is a sinusoid inshape. Each of the strain sensors produces a sinusoid which is phaseshifted by 2π/N radians where N is the number of blades on thepropeller. Thus, for example, in a propeller having 4 blades (N=4), thesinusoids are consecutively phase shifted by π/2 radians, or 90 degrees.The sinusoids may also be known as the “once-per-revolution cyclicresponse”, or “1P cyclic response”. For a propeller having healthy,identical blades, the sinusoids will have equal amplitude.

However, if a blade is damaged, the natural frequency at the first modeof the blade may change. Additionally or alternatively, the torsionalstiffness of the blade may change. Consequently, the 1P cyclic responsewill change compared to the other blades. The change which occurs may bean increase or a decrease of the blade dynamic magnification, dependingon the relationship between the first mode natural frequency and theonce-per-revolution forcing frequency.

For example, reduction in torsional stiffness of a blade due to damagewill result in increased blade twist magnification, which increases the1P cyclic response relative to the other blades. This can be seen as achange in amplitude of the sinusoid. Thus, by measuring a change inamplitude of the sinusoidal response of the strain sensors, damagedblade(s) can be identified.

It will be recognised that in this second propeller health monitoringarrangement, the strain measurements (i.e. the strain sensor outputs)are used directly. Bending moment is not calculated from these strainmeasurements.

FIG. 17 shows such a second arrangement, in which a propeller 510 isconnected to a drive shaft 530. The propeller 510 has a hub 550 havinghub arms 555, and four blades 520 a, 520 b, 520 c, 520 d (fourth blade520 d not shown), each extending from one of the hub arms 555, radiallyoutwardly from a rotational axis X of the propeller 510 and the driveshaft 530. Four strain sensors 540 a, 540 b, 540 c, 540 d (fourth sensor540 d not shown) are provided, each located on a front-facing surface ofa hub arm 555, such that strain sensors 540 a, 540 b, 540 c, 540 d arecircumferentially aligned with a respective blade 520 a, 520 b, 520 c,520 d. Thus sensor 540 a is aligned with blade 520 a, sensor 540 b isaligned with blade 520 b, sensor 540 c is aligned with blade 520 c andthe fourth sensor 540 d is aligned with the fourth blade 520 d (neitheris shown in FIG. 17). This arrangement is exemplary only and otherembodiments may have other numbers of blades, where some, preferably allof the blades each has a corresponding sensor located at a correspondinghub arm.

In use, the propeller drive shaft 530 will rotate in the usual way,thereby rotating the blades 520 a, 520 b, 520 c, 520 d and thecorresponding sensors 540 a, 540 b, 540 c, 540 d. The sensors measurethe strain in the hub arms 555 on which they are located, over time.

This measured strain is input to a processor (not shown) which may belocated in the FADEC or in the nacelle, in which cases the measuredstrains may for example be transmitted via a slip ring, telemetry and/orWi-Fi from the rotating part to the static part and then to the FADEC ornacelle. This allows for real-time processing of the measured strains.If it is desirable to instead analyse data after flight, it may also bepossible to record and store data and download this at the end of theflight.

The processor calculates the cyclic strain of the hub arms 555 from themeasured strain values for each of the sensors 540 a, 540 b, 540 c and540 d which corresponds to the cyclic response at an angular location ofeach respective blade 520 a, 520 b, 520 c and 520 d. The skilled personwould readily understand how the cyclic responses may be calculated fromthe measured strain values, in particular since the strain gauges areset up to record vibratory strain in micro strain. The cyclic responsecalculated for each sensor location is then analysed to determine if theblade associated with that sensor may be damaged, as is discussedfurther below.

FIG. 18 illustrates graphically 600 the cyclic blade response of thefour-blade propeller 510 of FIG. 17, based on the strain measurementsfrom the sensors 540 a, 540 b, 540 c and the fourth sensor 540 d (notshown in FIG. 17), over time. In this case, the blades of the propeller510 are healthy as will be discussed further below.

The solid line 610 corresponds to the cyclic blade response for a firstblade 520 a, as measured by a first sensor 540 a. The dotted line 620corresponds to the cyclic blade response for a second blade 520 b, asmeasured by a second sensor 540 b. The dot-dashed line 630 correspondsto the cyclic blade response for a third blade 540 b, as measured by athird sensor 540 c. The dashed line 640 corresponds to the cyclic bladeresponse for a fourth blade 520 d, as measured by a fourth sensor 540 d.

As is clear from FIG. 18, for four healthy blades, the cyclic bladeresponses 610, 620, 630, 640 are identical, depicted as sinusoids, witha phase shift of 90 degrees between each consecutive sinusoid due to thedifferent orientations of the blades and their respective sensors aroundthe propeller. In particular, the peak-to-peak amplitude 650 of each ofthe cyclic blade responses is the same.

Conversely, FIG. 19 is a copy of FIG. 18, but having superimposedthereon a further blade cyclic response 615, given by a dot-dot-dashedline, representing the case where there has been damage to the firstblade 520 a. In this exemplary scenario, the first blade 520 a has beendamaged and has a reduced torsional stiffness, resulting in increasedblade twist magnification, which increases the 1P cyclic responserelative to the other blades.

It can clearly be seen how the cyclic response 610 of the first blade520 a changes when it is damaged, i.e. the peak-to-peak amplitude of thecyclic strain measurements increases, as indicated by arrow 660. Arrow650 indicates the amplitude of the 1P cyclic response for healthyblades, compared to the exemplary 1P cyclic response for the damagedfirst blade 520 a, given by arrow 660.

Although FIG. 19 depicts an increase in the amplitude of the 1P cyclicresponse for a damaged blade, depending on the extent and type ofdamage, there may alternatively be a decrease in amplitude of the 1Pcyclic response for the damaged blade. Consequently, when monitoring theamplitudes of the 1P cyclic responses, the peak-to-peak amplitudes maybe compared and a percentage difference between the peak-to-peakamplitudes should be determined. The comparison of the peak-to-peakamplitudes may take the form:abs[(“650”−“660”)/“650”]

i.e. the absolute value of the difference between a healthy bladepeak-to-peak amplitude (650) and a damaged blade peak-to-peak amplitude(660) as a percentage of a healthy blade peak-to-peak amplitude (650).The numbers “650” and “660” refer to the peak-to-peak amplitudesindicated in FIG. 19. Irrespective of whether it is a percentageincrease or decrease in relative amplitude, if the magnitude of thatpercentage difference exceeds a threshold, then this is indicative thatthere is a damaged blade. The blade corresponding to the sensor that isproviding the 1P cyclic response with the irregular amplitude is thendetermined. Consequently, an alert for maintenance can be triggered andthe identified blade of the propeller can be inspected for damage andrepair or replacement work can be carried out. This method is termedherein the “hub method”, and will be described in more detail below withreference to FIG. 21.

FIG. 20 shows a second embodiment of a second propeller healthmonitoring arrangement, in which the propeller has an odd number ofblades, in this case three. Propeller 710 has a hub 750 with hub arms755 from which three blades 720 a, 720 b (not shown) and 720 c extend.The propeller is driven by a drive shaft 730 having a rotational axis Xcommon with the propeller 710. Mounted on the front facing surfaces ofthe hub arms 755 are strain sensors 740 a, 740 b (not shown) and 740 c,each strain sensor corresponding to a particular blade 220 a, 220 b and220 c respectively. The same hub method as has been described above (andwill now be described below with respect to FIG. 21) is applicable to apropeller having an even number or an odd number of blades.

FIG. 21 shows the “hub method” 800 for identifying damaged blades for apropeller with any number of blades. At step 810, strain gauges with afull-bridge arrangement are installed on the hub arms of the propeller,aligned with each blade.

At step 820, strain sensor data is analysed by a processor to determinethe cyclic strain. Various algorithms may be performed. A firstalgorithm may calculate the time taken for one revolution of thepropeller, given by (RPM/60)^−1. A second algorithm may record themaximum and minimum strains measured by each strain gauge sensor in eachrevolution. A third algorithm may calculate the cyclic strain amplitude(CycStr) in each revolution by calculating half of the peak-to-peakamplitude (i.e. (max−min)/2) using the recorded maximum and minimumstrains. A fourth algorithm may compare the cyclic strain amplitudes toeach other. This fourth algorithm may be carried out for example bycalculating the cumulative “error” in the cyclic strain amplitudemeasured by each strain gauge as given by the following formula:

${{error}\mspace{14mu}{CycStr}_{n}} = \left\lbrack {\sum\limits_{i = 1}^{N}\;\left( {{CycStr}_{n} - {Cycstr}_{i}} \right)^{2^{2}}} \right\rbrack^{0.5}$

where n is the reference number for the strain gauge sensor in questionand N is the total number of strain gauge sensors. From this, it can beestablished that the strain gauge which gives rise to the largest“error” in the cyclic strain amplitude has a corresponding blade whichmay be damaged.

At step 830, a decision is taken as to whether the cyclic strainamplitudes are equal within a defined tolerance. Thus if all of the“error”s calculated above are within a defined tolerance, such as forexample, a 3%, 5%, 10%, 15% or 20% tolerance, the blades are deemed atstep 840 to be healthy and the method returns to step 820. Otherwise, ifall of the “error”s calculated above are not within a defined tolerance,such as for example, a 3%, 5%, 10%, 15% or 20% tolerance, then themethod proceeds with step 850.

At step 850, it is established that the blade aligned with the sensorwhich has the largest “error” in cyclic strain amplitude compared to the“error” of the other sensors, or which has the largest difference incyclic strain amplitude compared to the cyclic strain amplitudes of theother sensors is damaged.

The identified blade can then be inspected for maintenance at a laterpoint and an alert for this can be raised.

Although the above first (sensors on shaft) and second (sensors on hubarms) arrangements have been described separately, it is also envisionedthat the two may be used in conjunction. For example, a propeller mayhave strain sensors both on the propeller hub arms and on the propellerdrive shaft. The bending moments of the shaft can be monitored accordingto one of the shaft methods described for the first arrangement above,using the sensors on the drive shaft. If the propeller has an evennumber of blades and so it is unclear from using the methodology of thefirst arrangement which blade of two diametrically opposed blades isdamaged, then the sensors on the hub arms could be used to identifywhich of the two diametrically opposed blades is damaged, in the mannerdescribed above for the second arrangement having hub arm sensors (i.e.the hub method).

The two above-described arrangements are not limited to propellershaving four blades, but may have any number of blades, including an oddnumber of blades.

The above embodiments describe the use of strain sensors. It will beappreciated that this term covers any sensor able to measure strain.Strain sensors may be referred to as strain gauges.

Typically, a strain gauge will measure a deformation (strain) as achange in electrical resistance. Such strain gauges may be termed “loadcells”.

A particularly useful type of strain sensor for use in embodiments ofthe disclosure is a full bridge strain gauge. Such strain gaugestypically utilise foil strain sensors. In a foil strain sensor, when anobject is deformed, the foil is deformed, causing its electricalresistance to change. Thus, the change in electrical resistance isdependent on the strain experienced. This change in resistance ismeasured in order to provide a measurement of the strain. In a fullbridge strain gauge, there are four active strain sensors (e.g. fourfoil strain sensors), which provides a very high sensitivity to bendingstrain. Furthermore, a full bridge strain gauge provides excellentsignal to noise ratio, rejects axial strain, compensates for temperatureeffects and compensates for lead wire resistance. However, half bridgestrain gauges may also be used, comprising two active strain sensors(e.g. two foil strain sensors), or quarter bridge strain gaugescomprising one active strain sensor. Such full, half and quarter bridgestrain gauges may be used as the “strain sensors” of the presentdisclosure. I.e., the foil sensors within such gauges would not beindividually used as a strain sensors, rather it is a gauge made fromfoil sensors that would be used as a strain sensor to implement thisdisclosure.

It will be appreciated that the methods and systems of the disclosure asdiscussed above provide significant advantages over prior art methodsfor assessing propeller health. The health of a propeller may bemonitored in real time during normal use of the aircraft, thus theaircraft does not need to be grounded for checks to be made. The methodis simple to implement, requiring only strain measurements to be madeusing readily available strain sensors, and a processor for carrying outthe processing steps. Thus, no complex equipment is required. Moreover,the processing required is not onerous, no complex models need to becreated or used as in some prior art methods. In the case of drive shaftstrain measurements, only simple calculations of steady bending momentand a comparison to a threshold is required. In the case of hub strainmeasurements, only the calculation of cyclic strain amplitudes and acomparison with other amplitudes is required. Thus, only minimalprocessing power is needed. Moreover, the particular blade pair, orindeed in certain embodiments the specific blade that is damaged can bepositively identified, thus enabling quick maintenance.

Additionally, the lifetime of the drive shaft can be improved, sincemaintenance of the damaged blades at the correct time can avoidprolonged steady bending of the drive shaft.

Furthermore, maintenance of the propeller can be restricted to thosepropeller blades which have been identified as being potentially damagedand unnecessary maintenance checks on healthy propellers need not becarried out.

Further aspects will be evident to the skilled person, in accordancewith the disclosure as defined in the claims.

The following numbered clauses set out features of the disclosure whichmay serve as basis for future amendments or divisional applications:

1. A method of monitoring the health of an aircraft propeller whilst thepropeller is in operation, the propeller having a plurality of bladesextending radially outwardly from a central axis extending through thepropeller and a propeller drive shaft, the method comprising: measuringthe strain in the propeller drive shaft using multiple primary strainsensors, each primary strain sensor providing a respective strainmeasurement; wherein the primary strain sensors are located around acircumference of the drive shaft of the propeller; and wherein eachstrain sensor is located such that it crosses a plane defined by theradial direction of a blade and the central axis, the plane beingbounded by the central axis.

2. A method as described in clause 1, further comprising measuringstrain in the propeller drive shaft using multiple secondary strainsensors, each secondary strain sensor being located around thecircumference of the drive shaft diametrically opposite to a respectiveprimary strain sensor and forming a sensor pair therewith.

3. A method as described in clause 1 or 2, further comprising: receivingmeasured strain data from each primary strain sensor or each strainsensor pair; and calculating a respective steady bending moment of thedrive shaft using the strain data received from each primary strainsensor or each strain sensor pair.

4. A method as described in clause 3, further comprising comparing themagnitude of the calculated steady bending moments to a threshold.

5 A method as described in clause 4, further comprising: establishingthat the health of the propeller may be impaired if the magnitude of thesteady bending moment of the drive shaft is above a threshold; andpreferably indicating an alert for maintenance if it is established thatthe health of the propeller may be impaired.

6. A method as described in clause 3, 4 or 5, further comprisingcomparing the magnitude of the calculated steady bending moments to oneanother.

7. A method as described in clause 6, further comprising establishingthat the health of the propeller may be impaired if the magnitude of oneof the steady bending moments of the drive shaft is outside of atolerance of the other steady bending moments of the drive shaft; andpreferably indicating an alert for maintenance if it is established thatthe health of the propeller may be impaired.

8. A method as described in clause 5 or 7, further comprising: for apropeller having an odd number of blades, identifying a damaged blade,by identifying the blade or blades corresponding to the primary sensoror sensor pair which has measured the strain data which has led to asteady bending moment being calculated which has a magnitude above thethreshold and/or which is outside of the tolerance of the other steadybending moments of the drive shaft; and preferably indicating an alertfor maintenance of the identified blade.

9. A method as described in clause 5 or 7, further comprising: for apropeller having an even number of blades, identifying which twodiametrically opposed blades may include at least one damaged blade, byidentifying the blades corresponding to the primary sensor or sensorpair which has measured the strain data which has led to a steadybending moment being calculated which has a magnitude above thethreshold and/or which is outside of the tolerance of the other steadybending moments of the drive shaft; and preferably indicating an alertfor maintenance of the identified blades.

10. A method as described in any preceding clause, wherein the strainsensors are full bridge strain gauges.

11. A method of monitoring the health of an aircraft propeller whilstthe propeller is in operation, the propeller having a plurality ofblades extending radially outwardly from hub arms of a propeller hub,which in turn extend radially outwardly from a central axis extendingthrough the propeller and a propeller drive shaft, the method comprisingmeasuring the strain in each of at least some of the hub arms usingstrain sensors, each of the strain sensors being provided on arespective hub arm.

12. A method as described in clause 11, wherein each strain sensor isprovided on an axially forward side of the hub.

13. A method as described in clause 11 or 12, wherein each strain sensoris circumferentially aligned with a propeller blade and is axiallyoffset from said propeller blade along a line parallel to the rotationalaxis of the propeller; preferably wherein each strain sensor is locatedradially inward of said propeller blade, and along a radial lineextending from a central axis of the propeller hub along the blade.

14. A method as described in clause 11, 12 or 13, further comprising:receiving the measured strain data from the strain sensors; calculatingamplitudes of the cyclic responses of the strain sensors using themeasured strain data from each of the sensors; and comparing theamplitude of at least one cyclic response to the amplitude of at leastone of the other cyclic responses.

15. A method as described in clause 14, further comprising establishingthat the health of a blade of the propeller may be impaired if theamplitude of the cyclic response measured by the sensor provided on thehub arm from which the blade extends is above or below the remainder ofthe amplitudes by at least 20%, preferably more than 15%, preferablymore than 10%, preferably more than 5%, and preferably more than 3%.

16. A method as described in clause 15, further comprising indicating analert for maintenance for a blade of the propeller if it is establishedthat the health of the blade may be impaired.

17. A method as described in any of clauses 12 to 16, wherein the strainsensors are full bridge strain gauges.

18. A method of monitoring propeller health comprising a combination ofthe methods of any of clauses 1 to 10 with the method of any of clauses11 to 17.

19. A system configured to perform a method for monitoring aircraftpropeller health as described in any preceding clause.

20. A propeller health monitoring system comprising: a plurality ofstrain sensor pairs configured to measure the strain in a hub arms of apropeller; and a processor configured to carry out the calculating,comparing and establishing steps as described in any of clauses 11 to18.

21. An aircraft propeller comprising the propeller health monitoringsystem as described in clause 20, wherein: the propeller has a pluralityof blades extending radially outwardly from hub arms of a propeller hub,which in turn extend radially outwardly from a central axis extendingthrough the propeller and a propeller drive shaft; and a strain sensoris mounted on each of at least some of the hub arms.

22. An aircraft propeller as described in clause 21, wherein theprocessor is integrated into a FADEC of the aircraft or in the nacelleand the strain sensors are configured to transmit the measured strain tothe processor via telemetry, Wi-Fi, or a slip ring.

23. An aircraft comprising an aircraft propeller as described in clause21 or 22.

24. A method of monitoring the health of an aircraft propeller whilstthe propeller is in operation, the propeller having a plurality ofblades extending radially outwardly from a central axis extendingthrough the propeller and a propeller drive shaft, the methodcomprising: obtaining measurements representative of strain in thepropeller drive shaft using multiple primary strain sensors, eachprimary strain sensor providing respective measurements representativeof strain; wherein the primary strain sensors are located around acircumference of the drive shaft of the propeller; wherein each primarystrain sensor is located such that it crosses a plane defined by theradial direction of a blade and the central axis, the plane beingbounded by the central axis; determining a respective steady bendingmoment of the drive shaft corresponding to each primary strain sensorusing the respective measurements representative of strain obtained byeach primary strain sensor; and establishing, based on the steadybending moments of the drive shaft, whether the health of the propellermay be impaired.

25. A method as described in clause 24, further comprising obtainingmeasurements representative of strain in the propeller drive shaft usingmultiple secondary strain sensors, each secondary strain sensor beinglocated around the circumference of the drive shaft diametricallyopposite to a respective primary strain sensor and forming a sensor pairtherewith; and wherein the step of determining a respective steadybending moment of the drive shaft additionally comprises utilising therespective measurements representative of strain obtained by eachsecondary strain sensor.

26. A method as described in clause 24 or 25, further comprisingindicating an alert for maintenance if it is established that the healthof the propeller may be impaired.

27. A method as described in clause 24, 25 or 26, wherein the step ofestablishing whether the health of the propeller may be impairedcomprises comparing the magnitude of the calculated steady bendingmoments to a threshold.

28 A method as described in clause 27, further comprising: establishingthat the health of the propeller may be impaired if the magnitude of acalculated steady bending moment of the drive shaft is above athreshold.

29. A method as described in clause 24, 25 or 26, wherein the step ofestablishing whether the health of the propeller may be impairedcomprises comparing the magnitude of the calculated steady bendingmoments to one another.

30. A method as described in clause 29, further comprising establishingthat the health of the propeller may be impaired if the magnitude of oneof the steady bending moments of the drive shaft is outside of atolerance of the other steady bending moments of the drive shaft.

31. A method as described in clause 28 or 30, further comprising: for apropeller having an odd number of blades, identifying a damaged bladeby: identifying the blade corresponding to the primary sensor or sensorpair which provided the measurement(s) representative of strain whichhas led to a steady bending moment being calculated which has amagnitude above the threshold and/or which is outside of the toleranceof the other steady bending moments of the drive shaft; and preferablyindicating an alert for maintenance of the identified blade.

32. A method as described in clause 28 or 30, further comprising: for apropeller having an even number of blades, identifying which twodiametrically opposed blades may include at least one damaged blade, by:identifying the blades corresponding to the primary sensor or sensorpair which has provided the measurement(s) representative of strainwhich has led to a steady bending moment being calculated which has amagnitude above the threshold and/or which is outside of the toleranceof the other steady bending moments of the drive shaft; and preferablyindicating an alert for maintenance of the identified blades.

33. A method as described in any of clauses 24 to 32, wherein the strainsensors are full bridge strain gauges.

34. A system configured to perform a method for monitoring aircraftpropeller health as described in any of clauses 24 to 32.

35. A propeller health monitoring system comprising: a plurality ofprimary strain sensors or pairs of primary and secondary strain sensors,the primary strain sensors or strain sensor pairs being configured toprovide measurements representative of strain in a drive shaft of apropeller; and a processor configured to carry out the determining,comparing and establishing steps as described in any of clauses 24 to33.

36. An aircraft propeller comprising the propeller health monitoringsystem as described in clause 35, wherein: the propeller has a pluralityof blades extending radially outwardly from hub arms of a propeller hub,which in turn extend radially outwardly from a central axis extendingthrough the propeller and a propeller drive shaft; the primary strainsensors or pairs of primary and secondary strain sensors are arrangedaround a circumference of the drive shaft of the propeller; each primarystrain sensor is located such that it crosses a plane defined by theradial direction of a blade and the central axis, the plane beingbounded by the central axis; and in the case in which strain sensorpairs are provided, each secondary strain sensor in the strain sensorpair is located around the circumference of the drive shaftdiametrically opposite to its corresponding primary strain sensor.

37. An aircraft propeller as described in clause 36, wherein theprocessor is integrated into a FADEC of the aircraft or in the nacelleand the strain sensors are configured to transmit the measured strain tothe processor via telemetry, Wi-Fi, or a slip ring.

38. An aircraft comprising an aircraft propeller as described in clauses36 or 37.

In the above described clauses 24 to 38, the step of determining arespective steady bending moment of the drive shaft may compriseconverting each measurement representative of strain into a bendingmoment value. This may be done by calculating the bending moment fromthe measurements representative of strain. A calibration may be madebetween measurements representative of strain and bending moment inorder to find a constant value to convert strain to bending moment,without the need for a full bending moment calculation.

The step of determining a respective steady bending moment of the driveshaft may include utilising a first algorithm to calculate the timetaken for one revolution of the propeller, given by (RPM/60)^−1. Then, asecond algorithm may be utilised to record the maximum and minimumbending moment determined utilising the measurements representative ofstrain from each primary sensor (or sensor pair) in each revolution. Athird algorithm may be used to calculate the steady bending moment (SBM)corresponding to each strain sensor in each revolution by taking theaverage of the recorded maximum and minimum bending moments, i.e.(max+min)/2.

The inventon claimed is:
 1. A method of monitoring the health of anaircraft propeller whilst the propeller is in operation, the propellerhaving a plurality of blades extending radially outwardly from a centralaxis extending through the propeller and a propeller drive shaft, themethod comprising: disposing multiple primary strain sensors around acircumference of the drive shaft of the propeller; and obtainingmeasurements representative of strain in the propeller drive shaft usingmultiple primary strain sensors, each primary strain sensor providing arespective measurement representative of strain; wherein each primarystrain sensor is located such that it crosses a plane defined by theradial direction of a blade and the central axis, the plane beingbounded by the central axis.
 2. The method as claimed in claim 1,further comprising: obtaining measurements representative of strain inthe propeller drive shaft using multiple secondary strain sensors, eachsecondary strain sensor being located around the circumference of thedrive shaft diametrically opposite to a respective primary strain sensorand forming a sensor pair therewith.
 3. The method as claimed in claim1, further comprising: determining a respective steady bending moment ofthe drive shaft using the respective measurements representative ofstrain obtained by each primary strain sensor.
 4. The method as claimedin claim 3, further comprising: comparing the magnitude of thecalculated steady bending moments to a threshold.
 5. The method asclaimed in claim 4, further comprising: establishing that the health ofthe propeller may be impaired if the magnitude of a calculated steadybending moment of the drive shaft is above a threshold.
 6. The method asclaimed in claim 3, further comprising: comparing the magnitude of thecalculated steady bending moments to one another.
 7. The method asclaimed in claim 6, further comprising: establishing that the health ofthe propeller may be impaired if the magnitude of one of the steadybending moments of the drive shaft is outside of a tolerance of theother steady bending moments of the drive shaft.
 8. The method asclaimed in claim 5, further comprising: for a propeller having an oddnumber of blades, identifying a damaged blade by: identifying the bladecorresponding to the primary sensor which provided the measurementsrepresentative of strain which has led to a steady bending moment beingcalculated which has a magnitude above the threshold; or for a propellerhaving an even number of blades, identifying which two diametricallyopposed blades may include at least one damaged blade, by: identifyingthe blades corresponding to the primary sensor which has provided themeasurements representative of strain which has led to a steady bendingmoment being calculated which has a magnitude above the threshold. 9.The method as claimed in claim 7, further comprising: for a propellerhaving an even number of blades, identifying which two diametricallyopposed blades may include at least one damaged blade, by: identifyingthe blades corresponding to the primary sensor which has provided themeasurements representative of strain which has led to a steady bendingmoment being calculated which is outside of the tolerance of the othersteady bending moments of the drive shaft; or for a propeller having anodd number of blades, identifying a damaged blade by: identifying theblade corresponding to the primary sensor which provided themeasurements representative of strain which has led to a steady bendingmoment being calculated which is outside of the tolerance of the othersteady bending moments of the drive shaft.
 10. The method as claimed inclaim 1, wherein the strain sensors are full bridge strain gauges. 11.An aircraft propeller system comprising: a propeller and a propellerhealth monitoring system configured to monitor the health of thepropeller while the propeller is in operation, the propeller healthmonitoring system comprising: a plurality of primary strain sensors orpairs of primary and secondary strain sensors, the primary strainsensors or strain sensor pairs being configured to provide measurementsrepresentative of strain in a drive shaft of a propeller; and aprocessor, wherein the propeller has a plurality of blades extendingradially outwardly from hub arms of a propeller hub, which in turnextend radially outwardly from a central axis extending through thepropeller and a propeller drive shaft; the primary strain sensors orpairs of primary and secondary strain sensors are arranged around acircumference of the drive shaft of the propeller; each primary strainsensor is located such that it crosses a plane defined by the radialdirection of a blade and the central axis, the plane being bounded bythe central axis; and in the case in which strain sensor pairs areprovided, each secondary strain sensor in the strain sensor pair islocated around the circumference of the drive shaft diametricallyopposite to its corresponding primary strain sensor.
 12. The aircraftpropeller system as claimed in claim 11, wherein the processor isintegrated into a FADEC of an aircraft or in a nacelle and the strainsensors are configured to transmit the measured strain to the processorvia telemetry, Wi-Fi, or a slip ring.
 13. A propeller aircraftcomprising the aircraft propeller system as claimed in claim
 11. 14. Theaircraft propeller system as claimed in claim 11, wherein the processoris configured to: determine a respective steady bending moment of thedrive shaft using the respective measurements representative of strainobtained by each primary strain sensor or using the respectivemeasurements representative of strain obtained by each strain sensorpair; and either compare the magnitude of the calculated steady bendingmoments to a threshold and establish that the health of the propellermay be impaired if the magnitude of a calculated steady bending momentof the drive shaft is above a threshold; or compare the magnitude of thecalculated steady bending moments to one another and establish that thehealth of the propeller may be impaired if the magnitude of one of thesteady bending moments of the drive shaft is outside of a tolerance ofthe other steady bending moments of the drive shaft.
 15. The method asclaimed in claim 2, further comprising: determining a respective steadybending moment of the drive shaft using the respective measurementsrepresentative of strain obtained by each strain sensor pair.
 16. Themethod as claimed in claim 15, further comprising: comparing themagnitude of the calculated steady bending moments to a threshold. 17.The method as claimed in claim 16, further comprising: establishing thatthe health of the propeller may be impaired if the magnitude of acalculated steady bending moment of the drive shaft is above athreshold.
 18. The method as claimed in claim 17, further comprising:for a propeller having an odd number of blades, identifying a damagedblade by: identifying the blade corresponding to the sensor pair whichprovided the measurements representative of strain which has led to asteady bending moment being calculated which has a magnitude above thethreshold; or for a propeller having an even number of blades,identifying which two diametrically opposed blades may include at leastone damaged blade, by: identifying the blades corresponding to thesensor pair which has provided the measurements representative of strainwhich has led to a steady bending moment being calculated which has amagnitude above the threshold.
 19. The method as claimed in claim 15,further comprising: comparing the magnitude of the calculated steadybending moments to one another.
 20. The method as claimed in claim 19,further comprising: establishing that the health of the propeller may beimpaired if the magnitude of one of the steady bending moments of thedrive shaft is outside of a tolerance of the other steady bendingmoments of the drive shaft.